1. Field Of The Invention
The present invention relates to a rod antenna and especially relates to a rod antenna which can receive AM and FM signals and can send signals in VHF and UHF bands. The present invention can be used for AM or FM radio, TV or any other radio communications, for example, MCA (Multi Channel Access) radio, wireless telephone, CB radio, Amateur radio and the like. The radios use AM, FM, VHF or UHF bands. Also, the present invention is used as a rod antenna for automobiles.
2. Description of the Prior Art
There are extendible and retractable antennas which can be stored in an automobile when the antenna is not used. For example, Japanese Kokai 59(1984)-30305 shows such a rod antenna. FIGS. 1 and 2 of Japanese Kokai 59(1984)-30305 show the elements 1a, 1b, . . . 1i of the rod antenna. FIG. 2 of the Kokai '305 corresponds to FIG. 9 of the present application. The lowest element 1a of the rod antenna is connected to the metallic base element 3 through an insulating material 2. The base element 3 is connected to the body 5 of an automobile by the fitting 4. The lowest element 1a is contacted by a spring 8 connected to the internal conductor 7 of the coaxial connector 6. The lowest portion 9 of the lowest element 1a is a stopper which prevents the element 1a from pulling out. The element 1i is connected to the flexible wire 10 which extends the elements of the rod antenna. In this type of rod antenna, there is a high electrostatic capacity between the lowest element 1a and the base element 3 because these two are close to each other. FIG. 3 of Japanese Kokai 59(1984)-30305 shows a high electrostatic capacity C between the antenna element and the ground. Although this type of antenna works well in a low frequency band such as AM or FM bands, this type of antenna may not work in high frequency bands such as VHF or UHF if the length of the antenna is adjusted because of the high electrostatic capacity. Therefore, in Japanese Kokai 59(1984)-30305, the lowest element of the rod is divided into two parts so that the antenna can work in a higher frequency band such as VHF or UHF. FIGS. 4 and 5 of Japanese Kokai 59(1984)-30305 show that the extensible lower element is divided into two parts 14a1 and 14a2. FIG. 5 of the Kokai '305 corresponds to FIG. 10 of the present application. Upper element 14a1 and bottom element 14a2 are connected through the center insulating material 15. A coaxial cable is inserted into bottom element 14a2. The center conductor 21 is connected to the upper element 14a1 and the external conductor 22 is connected to the bottom element 14a2. In accordance with this arrangement, the distance between the upper portion of the upper element 14a1 of the rod antenna and the ground is larger. Therefore the capacity is lower so the antenna can be used in a high frequency band.
However, when a rod antenna is connected to both an AM/FM band receiver and a VHF/UHF band transmitter in order to receive an AM/FM band signal and to send a VHF/UHF band signal, it is difficult to match impedances between the AM/FM band signal and the VHF/UHF band signal. Furthermore, if the power of the VHF/UHF band transmitter is relatively high, the AM/FM band receiver may be broken. Therefore, for example, when both an AM/FM radio and a VHF/UHF band transmitter are used in an automobile, separate antennas for each frequency band are required or a branching filter for separating the frequency signals is required.
Further, when a rod antenna is used for both an AM/FM band and a VHF/UHF band, an AM/FM power supply member may affect the high frequency band characteristics because the circuit for a high frequency band is very sensitive to its construction.
Japanese Kokai 61(1986)-46601 shows a first antenna connected to the coaxial cable for the first frequency band and a second antenna is made by using the external conductor of the coaxial cable and the body of an automobile for the second frequency band. However, this requires separate antennas for each frequency band. When an AM/FM band and a VHF/UHF band are used in one antenna, it is required for matching the AM/FM band and the VHF/UHF band to use a branching filter or a separator. In this way, as shown in FIG. 7, the frequency band width is relatively narrow. In FIG. 7, the graph shows the characteristics of a monopole antenna as to frequency vs SWR (Standing Wave Ratio). If SWR is under 2, an antenna is practical to use. In FIG. 7, a practical frequency band width is from about 905 to 955 MHz. For example, a mobile telephone has different frequencies for receiving and transmitting and there should be a certain distance between a received frequency band and a transmitted frequency. If a transmitted frequency band is set at 930 MHz, SWR for a received frequency is over 2. Therefore, it is difficult to receive a signal. In a conventional antenna, it is difficult to have a wide frequency bandwidth if an antenna is used for both AM/FM band and VHF/UHF band.